The accessory navicular is an extra bone typically situated at the plantar, medial, and/or proximal aspect of the navicular tuberosity in the foot. It is closely associated with the posterior tibial tendon (PTT).
Generally there have been considered three different types of an accessory navicular, but a fourth type has recently been described:
A type 1 is a small, round accessory bone at the lower aspect of the posterior tibial tendon without any contact to the navicular. This is most likely a sesamoid bone within the tibialis posterior tendon.
A type 2 is a large semilunar ossicle with close contact to the navicular separated from the latter by an irregular synchondrosis type joint.
A type 3 accessory navicular is one that is partially united with the navicular via a bony ridge, producing a “cornuate” process (which can get called os naviculare cornutum in Latin)
A type 4 is a large semilunar ossicle at the proximal aspect of the navicular tuberosity (like in type 2) and an additional bony extension at the distal aspect of the navicular tuberosity.
There can be a transition between types as the ossicle in a type 2 accessory navicular can develop a partial fusion and then be classified as a type 3. The classification of the type is important as that knowledge can help guide treatment. For example, it is the type 2 accessory navicular bones that become symptomatic and most often need treatment.